The study, from the journal Environment and Urbanization, says that the carbon emissions of cities vary widely.
"...some important trends emerge: low- and middle-income countries tend to have lower per capita emissions than high-income countries; dense cities tend to have relatively lower per capita emissions (particularly those with good transportation systems); cities tend to have higher emissions if in a cold climate zone. The most important observation is that there is no single factor that can explain variations in per capita emissions across cities; they are agglomerations of a variety of physical, economic and social factors specific to their unique urban life."
Jess Zimmerman at Grist sums it up:
"...while cities have the highest emissions per square mile, suburbs have far and away the highest emissions per person. Yeah, a single acre in New York is going to pollute more than an acre of strip mall in Scottsdale, but that's because everyone's stacked like Ikea cabinets. On a per-person basis, cities rule and strip malls drool."